1. Field of the Invention
This is a novel process for reducing the hydrolyzable chloride content of an isocyanatoalkyl ester of an organic carboxylic acid. In particular this process relates to the use of alkylene oxide or an epoxy resin as hydrogen chloride scavengers in such a process.
2. Prior Art
Isocyanatoalkyl esters of organic carboxylic acids can be prepared by reacting phosgene with various specific amines or nitrogen-containing compounds. British Pat. Nos. 1,280,007 and 1,252,099 and U.S. Pat. No. 2,821,544 describe different methods of preparation of these isocyanatoalkyl esters involving the phosgenation of nitrogen-containing compounds and are incorporated herein by reference. U.S. Pat. No. 2,718,516 discloses the preparation of 2-isocyanatoethyl methacrylate by a different reaction sequence and is also incorporated herein by reference.
The above-described methods of preparation all produce adducts of the isocyanatoalkyl esters and hydrogen chloride (believed to be carbamoyl chlorides) as co-products in addition to the esters themselves. These hydrogen chloride adducts contain hydrolyzable chloride, which is known to have an adverse impact on the reaction of the isocyanatoalkyl esters with hydroxyl, mercapto, amino and other active hydrogen functionalities.
Various techniques have been proposed in the literature to remove hydrolyzable chloride from specific isocyanates. U.S. Pat. No. 3,465,023 teaches that the use of an aqueous solution of a weak base reduces the hydrolyzable chloride content of an aliphatic isocyanate. Heating an organic isocyanate in the presence of an organic sulfonic acid or ester is disclosed in British Pat. No. 1,458,747 to remove hydrolyzable chloride-containing impurities. U.S. Pat. No. 3,793,362 describes a process for reducing the acidity and hydrolyzable chloride levels in polymethylene polyphenylisoycanates by treatment with a monomeric epoxide. However, only minor reductions in the hydrolyzable chloride level is attained by this epoxide treatment and no separation of the isocyanate compound from the residue of the epoxide compound is effected. Other methods for reducing the hydrolyzable chloride content of specific isocyanates are taught in U.S. Pat. Nos. 4,094,894; 4,118,286 and 4,146,550.
None of the aforementioned techniques for removing hydrolyzable chlorides are explicitly stated to be applicable to isocyanatoalkyl esters of organic carboxylic acids. Furthermore, some of these techniques are not operable with polymerizable isocyanates, such as 2-isocyanatoethyl methacrylate, because the high temperatures and pressures employed will effect substantial polymerization. The only method employed in the prior art to purify these isocyanatoalkyl esters is fractional distillation. Such fractional distillations are only moderately effective. In many utilities, the isocyanatoalkyl ester must contain less than 500 parts per million by weight of hydrolyzable chloride. Removal of this low residual levels of hydrolyzable chloride generally cannot be effected in a single fractional distillation.